As a display of a television, a personal computer, or the like, an active matrix-type liquid crystal display device capable of displaying a high-quality video image is used. A thin film transistor (hereinafter, called a “TFT”) is provided in each of pixel formation portions in a liquid crystal panel mounted on the liquid crystal display device. In the liquid crystal display device, when the TFT is in an on state, the potential of a video signal to be displayed is applied from a video signal line to a pixel electrode. Until the potential of a video signal to be displayed next is applied, the TFT is in an off state, and the potential of the video signal to be displayed is held in the pixel formation portion.
However, when the current passage time in the liquid crystal display device becomes long, the off characteristic of the TFT changes. As a result, in a liquid crystal panel of the normally black type (type in which the panel is seen black when no voltage is applied) having, for example, an n-channel type TFT, when a gate voltage is increased from a gate off voltage to a predetermined voltage which is lower than a threshold voltage in a state where the potential of the video signal is held in the pixel formation portion, although the TFT should still be in the off state, the brightness of the video image displayed in white decreases. In this state, when the TFT is turned on again and the potential of the video signal to be displayed next is applied to the pixel electrode, the video image becomes white again. At this time, the video image displayed in the liquid crystal panel looks gray to a viewer. The gate off voltage at which a video image to be essentially white is seen gray as described above is called a “blurring voltage”.
The blurring voltage decreases to a predetermined value as the current passage time of the liquid crystal display device increases. In consideration of the fact, the gate off voltage has to be set to a lower value. However, to enable the gate off voltage to be set to a lower value, the breakdown voltage of a scanning signal line drive circuit has to be set high, which causes a problem in that the manufacturing cost of the scanning signal line drive circuit becomes high.
The reason why the blurring voltage decreases as the current passage time of the liquid crystal display device increases is considered as follows. Specifically, when the current passage time of the liquid crystal display device increases, on the surface of a semiconductor layer serving as a channel region of the TFT, charges are accumulated in an interlayer insulating film near the surface of the side opposite to the side on which the gate electrode is disposed (hereinafter, called a “back gate side”). By charges accumulated in the interlayer insulating film, an inversion layer is formed on the surface of the semiconductor layer on the back gate side. As a result, since the gate-off voltage is applied, the off characteristic of the TFT which should essentially be in the off state changes, and the blurring voltage decreases. With the blurring voltage decrease, even if the gate-off voltage is applied to the TFT, a leak current flows from the drain electrode to the source electrode of the TFT. Consequently, the potential of the video signal held in the pixel formation portion decreases, and the brightness of the video image is reduced.
As the liquid crystal display device is spread in various fields such as in televisions, improvement in the display quality of the liquid crystal display device is required. Therefore, to increase the display quality, it is required to suppress decrease in the blurring voltage due to the change in the off characteristic of the TFT caused by increase in the current passage time.
Consequently, in Patent Document 1, a conductive plate connected to a pixel electrode or a conductive plate electrically isolated is conventionally disposed so as to face a channel region in a TFT via an interlayer insulating film.
In this case, charges are not easily accumulated on the surface of the interlayer insulating film and an alignment film, so that a change in the off characteristic of the TFT can be suppressed.
Prior Art Document
Patent Document
[Patent document 1] Japanese Unexamined Patent Publication No. 9-152628